Apparatus for Transforming The Aspect Ratio of An Optical Input Field Based On Stacked Waveguides

ABSTRACT

An apparatus consisting of stacked slab waveguides whose outputs are vertically staggered is disclosed. At the input to the stacked waveguides, the entrances to each slab lie in approximately the same vertical plane. A spot which is imaged onto the input will be transformed approximately to a set of staggered rectangles at the output, without substantial loss in brightness, which staggered rectangles can serve as a convenient input to a spectroscopic apparatus. A slit mask can be added to spatially filter the outputs so as to present the desired transverse width in the plane of the spectroscopic apparatus parallel to its dispersion.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/184,660, filed Jun. 5, 2009,entitled “Apparatus For Transforming The Aspect Ratio Of An OpticalInput Field Based On Stacked Waveguides,” which is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

This invention relates to the optical transformation of the shape of anominally circular object to an object with non-unity aspect ratiowithout substantial loss of brightness. When the aspect ratio of theoutput is chosen to be high, the resultant shape is particularlyappropriate as an input to a diffraction-grating based spectrometer.

2. Background and Relevant Art

When an object is illuminated with a beam of optical radiation for thepurpose of gathering light from the object, the optimum shape of theillumination beam is often nearly circular. If for example, the objecthas volumetric scattering properties, and it is desired to observe theback-scattered radiation, for an imaging system with limited field ofview, the optimum geometry for collecting the largest proportion ofback-scattered light is to image an area centered on a circularillumination beam.

If it is desirable to subject light which has either been scattered byor transmitted by an object to spectroscopic analysis, and thespectrometer is based on diffraction, it is advantageous to present anominally rectangular input to the spectrometer of high aspect ratio,where the short axis of the rectangle is in the direction of thedispersion of the spectrometer. Such an arrangement optimizes theresolution of the spectrometer. Hence, if it is advantageous to bothcollect light from an area having near unity aspect ratio and present ahigh aspect ratio input of nominally rectangular shape to thespectrometer, it is desirable to find an arrangement which opticallyconverts a near unity aspect ratio spot to a high aspect ratio output.There can, of course, be other circumstances besides spectroscopy wheresuch an optical conversion can be advantageous, and no limitation tospectroscopy for the usefulness of such a converter is implied.

The conversion of near-unity aspect ratio spots to high aspect ratiooutputs is sometimes accomplished with a fiber bundle, where the fibersare arrayed in a close packed geometry at the end where light is to becollected and re-arranged in a linear configuration at the opposite end.Such an arrangement will reduce the collection efficiency byapproximately the ratio of the area of the fiber cores to the area ofthe collection bundle. In addition, it is difficult to re-arrange theclose packed configuration at one end to the nominally linearconfiguration at the other end in a short length, hence it is difficultto make the fiber bundle devices compact.

Anamorphic prism pairs have been used for such transformations wherein anominally collimated beam passes through a first prism and is deflectedin angle, thereafter, passing through a second prism which is disposedat an appropriate angle with respect to the first. It is difficult toobtain changes in aspect ratio of more than a factor of five with thisarrangement and it is also very difficult to reduce reflection losses atall surfaces and in all polarizations because the beams can be incidentat high angles. Moreover, this approach requires a collimated beam whichmakes the embodiment less compact.

Another optical scheme for performing aspect ratio transforms is ananamorphic lens system as is described in “Anamorphic Condensing Opticsfor a Slitless Spectrograph”, J. R. Baskett, and I. D. Liu, AppliedOptics, 9, p. 49-52 (1970). The lens system consists of a sphericallens, followed by a cylindrical lens, followed by a second sphericallens, concluding with a second cylindrical lens. The system is complex,and is not suitable for high numerical aperture applications, and alsoconsumes considerable space.

In yet another approach a non-imaging elliptical concentrator isproposed in, “Elliptical Concentrators”, A. G. Garcia-Botella et al.,Applied Optics, 45, p. 7622-7627 (2006). The difficulty with suchconcentrators is that the angular distribution of the input radiation isnot preserved, with angular divergences in the two planes scalinginversely with geometrical scaling in the two planes. In manyapplications the input radiation diverges symmetrically and it isdesired that the output radiation nominally preserve this property.

Finally, it is possible to design a diffractive element that providesboth wavelength dispersion and transformation of aspect ratio. Such adesign is described in “Lensless anamorphic Fourier transform hologramrecorded with prism systems”, A. Viswanath and K. Srinivasan, AppliedOptics, 36, p. 5748-5755 (1997). It is difficult to simultaneouslyobtain high diffraction efficiency, wide field of view, and highnumerical aperture with this design.

BRIEF SUMMARY OF THE INVENTION

These and other limitations are addressed by the present invention,which discloses an apparatus whereby the aspect ratio of an opticalinput can be changed by a large factor without major sacrifice inbrightness, the resultant design also being highly compact.

In one embodiment, multiple slab waveguides having the form ofrhomboids, each having a different long dimension but similar widths andthickness are bonded together such that the long edges align. The inputsurfaces of the rhomboids are all contiguous. The radiation, upon beinglaunched in the slabs is reflected by a first 45° mirror which is oneangular face of the rhomboid. The reflected radiation propagates in thewaveguides and then encounters the second angular face of the rhomboidwhich is a second mirror also disposed at 45°. The light exits thesubsequent face of the rhomboids but since each rhomboid is a differentlength, the exit locations are off-set. The resulting output of eachrhomboid is roughly rectangular, and each such rectangle is offset fromthe others in the plane parallel to the short side of the rectanglesresulting in a staggered set of nominally rectangular outputs which issuitable for launching into a spectrometer. In particular, the inventiondisclosed is particularly advantageous when the aspect ratio of theoptical input differs by at least a factor of 2 from the aspect ratio ofthe optical output.

An alternate shape for the waveguide aspect ratio conversion device isalso disclosed, based on continuous bending rather than mirrors disposedat 45°.

A slit mask can be attached to the output faces of the rhomboids suchthat a portion of each rectangular output can be sampled, hence, theequivalent slit width of the spectrometer can be defined by this slitmask whereas otherwise it would be approximately the thickness of therhomboids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an isometric drawing of the stack of rhomboids that performthe transformation of the aspect ratio of an input to an output. FIG. 1Bis an exploded view of the assembly of rhomboids. FIG. 1C shows a singlerhomboid in two views and defines surfaces having importantfunctionality as well as the layers of the structure.

FIG. 2 shows an alternative shape for the waveguide aspect ratiotransformer based on continuous bends rather than mirrors.

FIG. 3A is an isometric drawing of the slit mask which can be attachedto the rhomboid assembly. FIG. 3B shows the input side of the slit maskwhich will receive the output from the rhomboid assembly and FIG. 3Cshows the opposite side of the slit mask from which the light will exit.

FIG. 4 is an isometric drawing of the rhomboid assembly attached to theslit mask by means of an attachment block which is bonded to both.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, an isometric drawing of one preferred embodimentis presented. In this example, five rhomboidal shaped slab waveguides10, 20, 30, 40, and 50 are attached to each other. An exploded view ispresented in FIG. 1B, where the width W, the thickness D, and thelength, L₁, of one waveguides 50 are defined. In this example thewaveguides 10, 20, 30, 40, and 50 all have the same widths, W, andthicknesses D. Their lengths are all different, and are defined asL_(1,2 . . . 5). The difference in lengths between any two contiguouswaveguides, L_(n)−L_(n-1), is approximately equal to W, such that eachprotrudes from its smaller neighbor at the output end by approximatelyW. In other examples, the volume of any of the slab waveguides is thevolume traversed by translating any quadrilateral, including but notlimited to a parallelogram or a trapezoid, in a direction perpendicularto the plane of the quadrilateral.

Two views of a single rhomboid waveguide are presented in FIG. 1C.Regions 13, 15, 17, 18, 19, and 21 are areas on the edges of therhomboid between the broad plane surfaces which have the shape ofrhombuses as shown in the front view of FIG. 1C. Light to be collectedis incident on the thin side of the rhomboid in region 13 and it isadvantageous to deposit an anti-reflection coating on the thin side inthis region. The region 17 is a mirror disposed nominally at 45° andreflects appreciably all the light propagating in the slab waveguide.The propagating light is next incident on mirror surface 18, whichreflects the light to output region 19. It is advantageous to deposit ananti-reflection coating in region 19.

The guiding material of the waveguide 11 is advantageously chosen to bea high index glass. In a particularly preferred embodiment, the index ofthe guiding material 11 is chosen to be sufficiently high such thatappreciably all the light incident on mirrors 17 and 18 istotally-internally reflected for the case of an air interface at thesesurfaces. Glasses with index of refraction greater than 1.8 are goodchoices for systems with low f-number. Identical claddings 12 aredeposited on either side of the guiding material of the waveguide 11 andare chosen from materials having an index of refraction lower than thatthe guiding material of the waveguide 11, such as silicon dioxide.Alternative choices for the guiding material depend on the wavelength ofradiation to be propagated. Optical plastics such as PMMA, polystyrene,or polycarbonate are possible in the visible and near IR regions.Silicon with low doping is advantageous at wavelengths in the region 1-6um. The general rule is to choose a material with low absorption for thewavelengths to be propagated. Other materials than can be used ascladdings 12 are adhesives such as epoxy or silicone. An air cladding isalso possible if space is provided between the slabs.

Regions 15 and 21 on the thin sides of the rhomboid can also be coatedwith a material whose index is lower than that of the guiding material11 to provide a cladding or they can be left uncoated, in which case anair interface provides the cladding. In a particularly preferredembodiment of this invention, an identical coating is applied to regions13 and 15, and an identical coating is applied to regions 19 and 21. Thecoating is designed such that light which is incident on regions 15 and21 that is within the desired range of propagation angles of thewaveguide undergoes total internal reflection whereas light which isincident on region 13 or which exits region 19 undergoes reducedreflection, the coating acting as an anti-reflection coating for therange of angles of the incident or exiting light. An example of aspecific implementation of the aforementioned embodiment is the casewhere the coating consists of a single material. To determine the properthickness of such a coating it is first noted that approximately ¼ wavethickness of a material whose index is intermediate between the mediumfrom which light is incident, and the medium into which light isentering can act as an anti-reflection coating over a range of anglesaround 0° with respect to the surface normal. It is then also noted thatit is possible to add integral multiples of ½ wave in thickness to the ¼wave-thickness coating which, for normally incident light will notincrease the reflection from the interface between the media. Finally,it is further noted that it is often desirable that a waveguide claddingbe thicker than ¼ wave, hence, by addition of integral multiples ofapproximately ½ wave thickness, a good cladding can be obtained inregions 15 and 21 whereas good anti-reflection properties can beobtained in regions 13 and 19. An example of a suitable coating would beapproximately 5/4 of a wave of Magnesium fluoride applied to a glass ofindex of roughly 1.80. When light is incident in a range of angles inregion 13 or exiting in a range of angles in region 19, the optimumthickness for a single layer to suppress reflections can be adjusted,the adjustment in general being in the range of less than ¼ wavethickness, hence, it remains possible to use the identical coating inregions 13 and 15, and the identical coating in regions 19 and 21. Aconsiderable simplification in the fabrication process thereby resultsas regions 13 and 15, or regions 19 and 21 can be coated simultaneouslywithout the use of a mask.

The widths, thicknesses, and number of slabs are chosen on the basis ofthe desired targets for the input and output. If the input has nearunity aspect ratio, and the number of slabs is n, then the thickness ofthe assembly is nD, which should be approximately equal to W, the width.If the output of the waveguide assembly is to be input to aspectrometer, the thickness D can be chosen such that the desiredresolution is obtained for the dispersion of a particular spectrometer.For example, if a spectrometer has dispersion 0.04 mm/nm and the desiredresolution is 2 nm, the thickness D is 0.08 mm.

The slab waveguides can be attached to each other by a direct bondingprocess such as is possible with silicon dioxide surfaces or with anoptical epoxy or silicone.

An isometric drawing of an alternate shape for the slab waveguides ispresented in FIG. 2, where two waveguides, 63, and 65 are shown asexemplars, it being understood that more than two can be deployed in theassembly. In this case, light enters input faces 68 and exits outputfaces 69. The direction of propagation of light is changed according tothe bend 67 in the waveguide. Turning of light can be accomplished withcombinations of mirrors and bends as desired.

There are cases where it may be advantageous to accept light from arestricted area of the output of the waveguide assembly. One example maybe where it is mechanically advantageous to use slabs of thickness Dgreater than that which would be acceptable for obtaining the desiredresolution, when the output of the waveguides are used as input for aspectrometer. In that case, it can be advantageous to mask off some partof the nominally rectangular output of each slab waveguide to reduce thedimension of the input in the plane of the dispersion of thespectrometer. An isometric drawing of a slit mask 70 is presented inFIG. 3A. The front side of the slit mask 70 is shown in FIG. 3B, whereasthe opposite surface is shown in FIG. 3C. The slit 80 shown on the frontside in FIG. 3B is intended to mask the output of the slab-waveguideassembly which will be brought into close proximity to the slit mask 70,the mask pattern being aligned to the outputs of the waveguides. Thewidth of the nominally rectangular opening 80 in the slit mask 70 ischosen to be less than D. A secondary mask 90 on the reverse side of theslit mask 70, is presented in FIG. 3C. The purpose of the secondary mask90 is to absorb back-propagating radiation in order to avoid multiplereflections between the slit mask and the detector. In addition, theslit pattern defines the transmittance angle of the mask in onedimension so as not to over-fill any subsequent spectrometer optics inthat dimension. The material in which the openings for the slit mask arecreated should be nominally opaque to the radiation. They are alsoideally chosen to have low reflectance as the slit is a reciprocalsurface to a detector in many spectrometers and it is often best toavoid multiple reflections between these surfaces. The substratematerial of the slit mask is a transparent material such as glass. It isadvantageous to deposit an anti-reflection coating on both surfaces ofthe substrate.

An isometric drawing of an assembly of slab waveguides 10, 20, 30, 40and 50 to a slit mask 70 is presented in FIG. 4. A block 100 is attachedboth to the slab-waveguide assembly, and the slit mask 70 to facilitatemanipulation during alignment between the two, and the subsequentbonding of the parts with an adhesive. A UV curing adhesive isconvenient for bonding the block 100 to the slit mask 70 in which casethe block 100 is fabricated from a material which is sufficientlytransparent to UV, such as glass. If it is desired that the output facesof the slabs not be in direct contact with the slit mask 70, a smallwell-defined spacing is advantageously obtained by using an adhesivewhich has been partially filled with microspheres of well controlleddiameter. Alternatively, tape of known thickness can be used between theblock 100 and the slit mask 70.

The output of the slab-waveguide assemblies presented is a staggeredpattern. The pattern can of course be rotated about its center such thatthe center of each segment is aligned vertically. If many slabs areused, an increasingly good approximation to a vertical object ofapproximately rectangular form can be obtained.

It is possible also to construct an output which is close to a singlerectangle if the waveguides are flexible. For example, the designpresented in FIG. 2 can have such an output if the waveguide 65 is bentin an S-curve configuration to align the output ends 69.

A staggered output can be beneficially used in spectroscopy if thedetector array of the spectrometer is two dimensional. In that case theassignment of wavelengths to each row of detectors can be distinct andaccount for the offset in the direction parallel to that of thedispersion. Also, it is possible to create a one dimensional detectorarray whose detectors have a shape similar to the staggered pattern. Asspectrometers often also have curvature of the contours of constantwavelength, this curvature can also be accounted for in shaping thedetectors.

Curvature of the constant wavelength contours of a spectrometer can alsobe compensated by shaping the output ends of the slab waveguide assemblysuch that they approximate this curvature. Such a shape can beadvantageously created by molding.

Although the detailed description contains many specifics, these shouldnot be construed as limiting the scope of the invention, but merely asillustrating different examples and aspects of the invention. It shouldbe appreciated that the scope of the invention includes otherembodiments not discussed in detail above. Various other modifications,changes and variations which will be apparent to those skilled in theart may be made in the arrangement, and details of the apparatus of theinvention disclosed herein without departing from the spirit and scopeof the invention.

1. An apparatus for converting a shape of an optical input to adifferently shaped optical output, comprising: at least two slabwaveguides wherein a location light is input along a portion of one sideof each of the waveguides is distinct from a location light is outputfrom one side of each of the waveguides; wherein there is a geometricaldifference between each waveguide such that the location light is outputfrom each of the waveguides is displaced in at least two directions withrespect to each other, or the location light is input to each of thewaveguides is displaced in at least two directions with respect to eachother; wherein an aggregate area of the locations light is input to thewaveguides corresponds approximately to the shape of the optical input;and wherein an aggregate area of the locations light is output from thewaveguides corresponds approximately to a desired shape of opticaloutput, wherein the desired shape of the optical output is differentfrom the shape of the optical input.
 2. The apparatus of claim 1,wherein the volume of any of the slab waveguides is the volume traversedby translating a quadrilateral in a direction perpendicular to the planeof the quadrilateral.
 3. The apparatus of claim 2, wherein thequadrilateral is a parallelogram.
 4. The apparatus of claim 2, whereintwo of the angles between respective adjacent edges in the plane of thequadrilateral are substantially 45°.
 5. The apparatus of claim 4,wherein the at least two slab waveguides are rhomboids, each having adifferent long dimension.
 6. The apparatus of claim 2, where at leastone side perpendicular to the plane of the quadrilateral is a mirror. 7.The apparatus of claim 6, wherein at least some light propagating in thewaveguide undergoes total internal reflection at the mirror.
 8. Theapparatus of claim 2, wherein the locations light is input and output ofthe waveguides are antireflection coated.
 9. The apparatus of claim 8,wherein the coating of the locations light is input and output of thewaveguides extends substantially along an entire length of those sidesand acts as a waveguide cladding in areas distinct from the locationslight is input and output.
 10. The apparatus of claim 9, wherein thecoating has a thickness substantially of ¼ wavelength plus an integralnumber of half wavelengths of the input light in an index of the coatingmaterial.
 11. The apparatus of claim 1, wherein a guiding material ofthe waveguides is glass with an index greater than 1.80.
 12. Theapparatus of claim 1, wherein an aggregate area of the locations lightis output from the waveguides is further restricted by a mask.
 13. Theapparatus of claim 1, wherein the outputs from the waveguide are curvedso as to compensate for a curvature of constant wavelength contours in aspectrometer to which the apparatus inputs light.
 14. The apparatus ofclaim 1, wherein one or more sides of the slab waveguides are curved,wherein a form of the curvatures differs between waveguides so as toproduce a displacement either in the location light is output from eachof the waveguides with respect to each other, or in the location lightis input from each of the waveguides with respect to each other.
 15. Theapparatus of claim 1, wherein the at least two slab waveguides arestacked together.
 16. The apparatus of claim 1, wherein the shape of theoptical input is nominally circular, and the desired shape of theoptical output is a staggered set of nominally rectangular outputs fromthe waveguides.
 17. The apparatus of claim 1, wherein the ratio of theaspect ratio of the optical input to the aspect ratio of the opticaloutput is less than 0.5.
 18. The apparatus of claim 1, wherein the ratioof the aspect ratio of the optical input to the aspect ratio of theoptical output is greater than 2.0.